Bond Parameters: Meaning, Bond Length, Angle, Enthalpy

Bond Parameters: Atoms combine together in different forms to form molecules. The constituent atoms of a molecule are held together by a certain force. This force is attractive in nature and holds various constituents (atoms, ions, etc.) together in different chemicals to form a chemical bond. There are basically three different types of chemical bonds. These are ionic or electrovalent, covalent, and coordinate bonds. Every bond has some characteristic feature associated with it known as bond parameters. Let’s learn about these parameters and their effect on chemical bonding in this article.

Bond Parameters: Overview

Covalent bonds are characterised on the basis of various parameters known as bond parameters. These parameters offer insight into the stability of a chemical compound and the strength of the chemical bonds holding its atoms together.

The bond parameters are:

1. Bond Length
2. Bond Angle
3. Bond Enthalpy
4. Bond Order

Bond Length

The equilibrium distance between the nuclei of two bound atoms in a molecule is defined as bond length. It is equivalent to the sum of the two bound atoms’ covalent radii. The covalent radius is the radius of an atom’s core that is in contact with the core of an adjacent atom in a bonded situation. It is half of the distance between two similar atoms joined by a covalent bond in the same molecule.

The bond length in a covalent molecule \({\rm{AB}}{\rm{. }}{{\rm{r}}_{\rm{A}}}\) and \({{\rm{r}}_{\rm{B}}}\) are the covalent radii of atoms \({\rm{A}}\) and \({\rm{B}}\) respectively. The bond length, \({\rm{R = }}{{\rm{r}}_{\rm{A}}}{\rm{ + }}{{\rm{r}}_{\rm{B}}}\)

Bond length is measured by:

1. Rotational spectroscopy
2. \({\rm{X – }}\)ray diffraction
3. Neutron diffraction

The bond length varies inversely to the bond order. The shorter the length of the bond, the higher is the bond order, stronger is the force of attraction in between the bonding atoms. However, the bigger the size of the atom, the longer is the bond length.

Bonded atoms absorb thermal energy from their surroundings that cause them to vibrate. This vibration causes the bond length to vary. Hence, the bond length of a covalent bond represents the average distance between the nuclei of the participating atoms.

The factors on which bond length depends:

1. Bond Multiplicity: With an increase in bond multiplicity, the bond length decreases.
2. Size of an Atom: The length of a bond is proportional to the size of the atom. With an increase in the size of the atoms, the bond length increases.

Periodic Trends in Bond Length

Bond lengths are directly proportional to the atomic radii of the constituting atoms. On moving along the period, the atomic radii decrease however, down the group atomic radii increase. Hence, bond length decreases across the period and increases down the group.

Bond Angle

The angle between the atomic orbitals containing bonding electron pairs around the central atom in a molecule/complex ion is called the bond angle. In simple words, it is the geometric angle between any two adjacent covalent bonds. It is measured in degrees. This bond parameter determines the molecular geometry of a compound.

The bond angle in a water molecule is shown below:

The bond angle helps in determining the shape of the molecule as it gives us a rough idea regarding the distribution of orbitals around the central atom in a molecule/complex ion.

Bond Energy or Bond Enthalpy

Bond energy is defined as the amount of energy required to break one mole of a particular type of bond between two atoms in the gaseous state. This bond parameter measures the strength of a chemical bond. The unit of bond enthalpy is \({\rm{KJ}}\,{\rm{mo}}{{\rm{l}}^{ – 1}}.\) Larger the bond enthalpy, stronger will be the bond in the molecule. The electronegativity difference between the participating atoms in the chemical bond also contributes to the bond energy.

Bond energy or enthalpy is not the same as bond dissociation energy. Bond enthalpy is the energy associated with the homolytic cleavage of a bond whereas bond dissociation energy is the average of the bond dissociation energies of all bonds (of a specific type) in a molecule.

The average bond enthalpy is calculated by dividing the total bond dissociation enthalpy by the number of bonds broken.

Factors Affecting Bond Energy

The amount of energy required to break a chemical connection determines its strength. As a result, bond energy is:

1. Bond energies are inversely related to bond length, meaning that longer bonds have lower bond energy.
2. Multiple bonds have large bond energy because they are directly proportional to the bond order.
3. The atomic radii of the atoms that make up the bond are inversely proportionate (since the atomic radius is directly proportional to bond length).

Bond Order

Bond order is defined as the total number of covalently bonded electron pairs present between two atoms in a molecule. It can be found by counting the total number of electron pairs present between the atoms in the Lewis structure of the molecule.

1. The bond order of a molecule having a single shared electron pair is \(1,\) i.e Single bonds have a bond order of \(1.\) Example: \({{\rm{H}}_{\rm{2}}}\) (with one shared electron pair) and in \({{\rm{N}}_{\rm{2}}}\) (with three shared electron pairs) is \(1\) and \(3\) respectively.
2. The bond order of a molecule having two shared electron pairs is \(2,\) i.e, Double bonds have a bond order of \(2.\) Example: \({{\rm{O}}_{\rm{2}}}\)
3. The bond order of a molecule having three shared electron pairs is \(3,\) i.e, Triple bonds have a bond order of \(3.\) Example: \({{\rm{N}}_{\rm{2}}}\)
4. If the bond order of a covalent bond is \(0,\) then the two atoms are not covalently bonded (no bond exists).
5. The nitrogen-oxygen bond in the nitrate ion has a bond order of \(4/3\) or \(1.33.\) It’s computed by multiplying the total number of nitrogen-oxygen bonds \((4)\) by the total number of nitrogen-oxygen groups that are covalently bonded \((3).\) The bond order of isoelectronic species (those species that have the same number of electrons) is the same. Considering, for example, \({{\rm{N}}_2},{\rm{N}}{{\rm{O}}^ + }\) and \({\rm{CO}}\) have a total of \(14\) electrons and all of them have an equal bond order of \(3.\)
6. The greater the order of the bond, shorter is the bond length, the greater is the stability of molecules.

Bond Order as per the Molecular Orbital Theory

The bond order of a covalent bond is equal to half of the difference between the number of bonding and antibonding electrons, according to molecular orbital theory. The following formula is used to express this:

Summary

Bond parameters are vital to the characterisation of covalent bonds. These parameters provide an insight into the strength, geometry, stability, and reactivity of the bonds present between atoms. It also gives an idea about the multiplicity of the bonds and the energy required for bond cleavage. In this article, we learned the various bond parameters such as bond length, bond energy, bond order, and bond angle.

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Frequently Asked Questions (FAQs) on Bond Parameters

Q.1. What is meant by bond angle?
Ans: The angle between the atomic orbitals containing bonding electron pairs around the central atom in a molecule/complex ion is called the bond angle. In simple words, it is the geometric angle between any two adjacent covalent bonds. It is measured in angles. This bond parameter is crucial to the determination of the molecular geometry of a compound.

Q.2. Which bond is the strongest bond?
Ans: The ionic bond is the strongest among all chemical bonds. In such bonding, each of the two atoms is converted into cations and anions that bind them together.

Q.3. Out of double bond and triple bond, which one is stronger?
Ans: Triple bonds are stronger than double bonds due to the presence of two π bonds rather than one.

Q.4. What is the enthalpy of bond dissociation?
Ans: The amount of energy required to break one mole of a particular type of bond between two atoms in the gaseous state is called the bond dissociation enthalpy. This bond parameter measures the strength of a chemical bond. The unit of bond enthalpy is kJ/mol. The larger the bond enthalpy, the stronger will be the bond in the molecule.

Q.5. What is the bond order of \({\rm{CO}},\) according to MOT?
Ans: \({\rm{CO}}\) has \(14\) electrons, bonding electrons\(=10\) and antibonding electrons\(=4.\)
\({\rm{Bond}}\,{\rm{order}} = \frac{{{\rm{ total}}\,{\rm{no}}{\rm{.}}\,{\rm{of}}\,{\rm{bonding}}\,{\rm{electrons}} – {\rm{total}}\,{\rm{no}}{\rm{.}}\,{\rm{of}}\,{\rm{anti}}\,{\rm{bonding}}\,{\rm{electrons }}}}{2}\)
\({\rm{Bond}}\,{\rm{Order = }}\frac{{{\rm{10 – 4}}}}{{\rm{2}}}{\rm{ = 3}}\)
Hence, the bond order of \({\rm{CO}}\) is \(3,\) i.e, it has a triple bond between carbon and oxygen atom.